Elevator systems are typically guided between a pair of ferrous rails, such as steel, which are also used as braking surfaces for emergency stops. In normal operation, all of the motion of the elevator and all of the arresting of that motion is caused by the hoist ropes, which are moved upwardly and downwardly, or held in a fixed position by means of a sheave, the motion of the sheave being controlled by the elevator drive motor and the machine brake which are mechanically coupled to the sheave. Machine brakes typically are spring actuated into the braking position against a drum or a disk attached to the sheave, and use electromagnets to release the brakes from the braking position when the elevator is to move. This provides fail-safe braking insofar as electrical power or electronic signaling is concerned.
In a typical elevator system a governor rope is attached to the elevator and rotates a governor, at a rate of rotary speed that relates to the elevator's linear speed, which has fly weights that move outwardly with increasing speed as a result of centrifugal force. When the elevator exceeds a predetermined speed by some small percent, the fly weights will be displaced sufficiently outward to trip an overspeed switch and release a latch which allows a jaw to grip the governor rope and arrest its motion. The arrested governor rope causes actuators to pull safety rods on the elevator car causing the operation of safety brakes (sometimes called "safeties"), which are typically wedges that become jammed between a safety block and opposite sides of the of the elevator guide rail causing an increasing frictional force which abruptly stops the elevator car.
German patent, No. 198,255 suggested using electromagnets as an elevator safety brake, which would engage as a result of cable breakage, slackening of cable tension or exceeding predetermined speeds. Braking action is due both to mechanical friction and electromotive force generated in the car's guidance rail. A battery is used, and the operational capability of the system is tested with a switch each time that the elevator comes to rest. Similar eddy current braking systems have been devised for railroad trains, one example of which is shown in a pamphlet entitled "Eddy Current Brake WSB", published by Knorr-Bremse GMBH, 1975. The system described therein has electromagnets of alternating polar orientation dispersed above a length of track, on a carrier which hangs directly from the railway car truck. The magnets are kept suspended away from the rails by pneumatic cylinders except when emergency braking is desired; then, the air pressure is released so that the brake can drop down on the rail, thereby providing frictional braking action as a consequence of the electromagnetic attraction of the electromagnets to the rail, as well as magnetodynamic braking as a consequence of eddy currents induced by the alternating magnetic poles traversing the material of the track.
Other prior art elevators use a passive magnetodynamic car safety brake having permanent magnets arranged with alternate magnetic polarity. As the magnets pass a ferrous member an electromotive field is produced. The safety brake operates safety rods pulling a brake shoe arrangement into engagement with a surface used for braking. Such systems can provide safety braking action for either direction of travel of the elevator car. In this particular embodiment the need for a rope assembly governor is eliminated.
Another overspeed brake of the prior art which does not require a rope assembly governor uses a magnet mounted on the elevator which induces an eddy current in the conductive vane which in turn produces an electromagnetic reaction force on the magnet, causing the magnet to actuate a brake, thereby braking the elevator car at any vertical point between the hoist way terminals.